The present invention relates to a work-contact probe head wherein a probe member is forced by restoring forces into a support which determines its zero position, from which support it is displaceable relative to its mount in one direction in space and can be tilted in all directions with respect to this direction of displacement.
Such probe heads are described, for example, in West German Pat. No. 2,347,633 or West German Offenlegungsschrift OS No. 2,743,665, and are used in coordinate measuring instruments, preferably multi-coordinate measuring instruments; and they serve to produce a signal when the probe element, i.e., the movable part of the probe head, contacts the workpiece which is to be measured.
To produce this signal, switch elements are frequently integrated into a support which determines and references the zero position of the probe element, and the switch elements respond upon deflection of the probe element out of its support-referenced position. Other probes are also known, for example, from West German Pat. No. 2,712,181, in which production of the signal is independent of probe-element deflection and in which, therefore, the support which determines the zero position serves merely as a flexible knee point to prevent the probe element from being deformed during the course of a work-probing procedure.
In all cases, precision of the probe head used for the coordinate measurement depends on how well the zero position of the probe element is reproduced upon return into its support after each work-contacting procedure.
In the above-mentioned known probe heads, the support consists of self-centering three-point abutments fixed to the housing of the probe head, the probe element being spring-urged into contact with the abutments. To enable probe-element displacement in all six directions in space, the probe element is frequently indirectly mounted on an intermediate ring, and the latter in turn is mounted (via another three-point reference abutment) to the part which is fixed to the housing; the intermediate ring is preloaded against the housing reference by a spring force which is in the direction opposite to the direction of resiliently urging the probe element to its referencing engagement with the intermediate ring.
The involved three-point supports provide V-shaped depressions into which balls or cylinders have sliding engagement. The sliding friction which characterizes these supports is not negligible; it impedes return of the probe element into its precise zero position and therefore limits the measurement precision of coordinate measuring instruments which are thus equipped.
This friction problem is also present in the case of the probe head described in West German Pat. No. 2,365,984, wherein the probe element comes to rest on a simple three-point support which is not self-centering and which has separate means for centering the probe element and for assuring against torque about the longitudinal axis of the probe element. The probe head described in West German Offenlegungsschrift OS No. 2,248,967, wherein the probe element is centered by a plurality of tensed wires, is also characterized by relatively great friction in the at-rest or zero position, since the probe element comes to rest on a part fixed to the housing via an annular resting surface.